The present invention relates to a beam splitting element for a single reflex lens digital camera.
Recently, digital cameras have become widely used instead of cameras using silver-salt films. Among such digital cameras, an SLR (single lens reflex) type digital camera is advantageous since the image of the object formed by the photographing lens of the camera is observed through the finder, no parallax is generated between the image captured by an image capturing element such as a CCD (Charge Coupled Device), and the image observed through the finder.
An example of the SLR digital camera is provided with a beam splitter which splits light passed through the photographing lens into light directed to the image capturing element and light directed to the finder optical system. The beam splitter includes a beam splitting surface (i.e., a half mirror surface), which may be formed with a multi-layer film or coating made of dielectric material. The multi-layer film is generally designed to exhibit optimum reflecting/transmitting characteristics for the visible light, whose wavelength range is, for example, approximately from 400 nm to 700 nm.
FIG. 1 shows an example of a conventional photographing optical system 10xe2x80x2 for an SLR digital camera employing a beam splitting element 2xe2x80x2 and a CCD 3. The beam splitter 2xe2x80x2 is provided with a half mirror surface 2xcex1xe2x80x2. The half mirror surface 2xcex1xe2x80x2 is provided with a multi-layer film which is designed to have an optimum reflectivity (e.g., 30%) for the visible light incident thereon at a predetermined incident angle (e.g., 45xc2x0).
FIG. 2 shows a graph indicating a relationship between the reflectivity with respect to the wavelength of a beam incident on the half mirror 2xcex1xe2x80x2 at the incident angles of 45xc2x0 and (45xc2x110)xc2x0.
As shown in FIG. 2, when the incident angle of the beam is 45xc2x0, the beam is reflected at the reflectivity of 30% substantially at any wavelength within the visible range (i.e., 400 nm through 700 nm). However, light from the object includes a beam which is inclined with respect to the optical axis of the photographing optical system. Generally, the incident angle of such a beam with respect to the half mirror surface 2xcex1xe2x80x2 is within a range approximately from 35xc2x0 to 55xc2x0. The characteristics of the beams incident on the half mirror surface 2xcex1xe2x80x2 at the incident angles of 35xc2x0 and 55xc2x0 are also indicated in FIG. 2. As shown in FIG. 2, the reflectivity characteristics vary depending on the incident angle. Specifically, when the incident angle is lowered with respect to a designed angle (i.e., 45xc2x00), the characteristic shift in the longer wavelength side (i.e., right-hand side in FIG. 2), while if the incident angle increases, the characteristic shift in the shorter wavelength side. If a wavelength range corresponding to the maximum reflectivity (i.e., 30% in FIG. 2) shifts as the incident angle changes, the color of an object cannot be captured accurately. For example, if the incident angle decreases, components having shorter wavelengths (e.g., 400 nm) are reflected by the half mirror surface 2xcex1xe2x80x2 at a lower reflectivity. Accordingly, the reflected light includes less lower-wavelength components than the light from the object. This also causes the light transmitted through the half mirror surface 2xcex1xe2x80x2 to include more lower-wavelength components. Similarly, if the incident angle increases, components having longer wavelengths (e.g., 700 nm) are reflected at a lower reflectivity. In such a case, the reflected light includes less higher-wavelength components than the light from the object, and the light transmitted through the half mirror surface 2xcex1xe2x80x2 includes more higher-wavelength components than the light from the object. If such a phenomenon occurs, a part of an image captured by the CCD and/or observed through the finder appears reddish or bluish, which is different from the color of the object viewed by the naked eyes.
Conventionally, for the image captured by the CCD, an improved image processing system is provided in the digital camera to compensate for the shift of the reflectivity characteristics so that the change of the color of the image captured by the CCD and displayed on an LCD is not conspicuous. However, such a high-performance image processing system increases the manufacturing cost. Further, since a longer image processing time duration is required, movement of the object may not be viewed in real time through the LCD.
Further, with respect to the image observed through the finder, since the optical image is directly viewed, the image cannot be compensated. Thus, the operator is required to observe the image including the above-described defects, which may cause the operator to feel uncomfortable.
It is therefore an object of the invention to provide an improved beam splitting element which retains a predetermined reflectivity characteristic for the visible light including components having predetermined incident angle range.
For the above object, according to the invention, there is provided a beam splitter for a digital camera for capturing an image of an object using an image capturing element, the beam splitter splitting light passed through a photographing lens of the camera into a beam directed to the image capturing element and a beam directed to a finder optical system of the camera, which is provided with at least one optical element provided with a beam splitting surface that is inclined with respect to an optical axis of the photographing lens, and a multi-layer film formed on the beam splitting surface, the multi-layer film including a plurality of layers of dielectric materials, the multi-layer film being formed such that the beam splitting surface of the at least one optical element exhibits a substantially constant reflectivity at least for visible light that is incident on the beam splitting surface at any incident angle within a predetermined range.
With this structure, all the beams incident on the beam splitting surface are split thereby at a substantially same ratio, and therefore the defects in the conventional beam splitter can be overcome.
Optionally, the multi-layer film may be formed such that a layer of a dielectric materials having a relatively high refractive index and a layer of a dielectric material having a relatively low refractive index are alternately layered. In particular, the low refractive index may be defined to fall within a range from 1.30 to 1.66, and the high refractive index may be defined to fall within a range from 1.90 to 2.50.
In a specific structure, the at least one optical element comprises first and second right-angle prisms arranged from a photographing lens side, inclined surfaces of the first and second prisms being adhered to each other with the multi-layer film sandwiched therebetween, the inclined surface of the second prism being the beam splitting surface.
In the above structure, the multi-layer film may be formed such that the beam splitting surface exhibits a substantially constant reflectivity for light whose wavelength is within a range broader than the wavelength range of the visible light in either a lower wavelength direction or a higher wavelength direction, and which light is incident on the beam splitting surface at a predetermined incident angle.
Optionally, the multi-layer film may include at least two types of dielectric layers respectively having a relatively low refractive index and a relatively high refractive index, the two types of dielectric layers being alternately layered.
Also in this case, the low refractive index may fall within a range from 1.30 to 1.66, and the high refractive index may fall within a range from 1.90 to 2.50.
In a specific example, the multi-layer film includes six layers L1-L6 respectively having the low refractive indexes and seven layers H1-H7 respectively having the high refractive indexes. The layers L1-L6 and H1-H7 are arranged, from the light incident side, in the order of H1, L1, H2, L2, H3, L3, H4, L4, H5, L5, H6, L6 and H7, and a quarter-wavelength optical thickness (QWOT) of each of the layers H1-H7 and L1-L7 is defined as follows:
143 nm less than QWOT of H1 less than 175 nm;
172 nm less than QWOT of L1 less than 211 nm;
1304 nm less than QWOT of H2 less than 1594 nm;
182 nm less than QWOT of L2 less than 223 nm;
204 nm less than QWOT of H3 less than 249 nm;
463 nm less than QWOT of L3 less than 565 nm;
928 nm less than QWOT of H4 less than 1134 nm;
450 nm less than QWOT of L4 less than 550 nm;
835 nm less than QWOT of H5 less than 1021 nm;
464 nm less than QWOT of L5 less than 567 nm;
498 nm less than QWOT of H6 less than 609 nm;
179 nm less than QWOT of L6 less than 218 nm; and
480 nm less than QWOT of H7 less than 586 nm.
Optionally, the predetermined range is a range of xc2x115 degrees with respect to a predetermined angle. The predetermined angle may be 45 degrees, and this may be varied depending on the arrangement of the beam splitter with respect to an optical axis of the photographing lens.
In an another case, the multi-layer film may be arranged on the photographing lens side of the at least one optical element, the photographing lens side of the multi-layer film being exposed to a predetermined medium having a refractive index lower than that of the at least one optical element.
Optionally, the multi-layer film includes at least two types of dielectric layers respectively having a relatively low refractive index and a relatively high refractive index, the two types of dielectric layers being alternately layered.
Also in the above case, the low refractive index may fall within a range from 1.30 to 1.66, and the high refractive index may fall within a range from 1.90 to 2.50.
In a specific example, the multi-layer film includes two layers L1 and L2 respectively having the low refractive indexes and two layers H1 and H2 respectively having the high refractive indexes, the layers H1, L1, H2 and L2 being arranged, in this order, from the light incident side; and
wherein a quarter-wavelength optical thickness of each layers H1, H2, L1 and L2 are defined as follows:
566 nm less than QWOT of H1 less than 691 nm;
314 nm less than QWOT of L1 less than 384 nm;
71 nm less than QWOT of H2 less than 87 nm; and
308 nm less than QWOT of L2 less than 376 nm.
Optionally, at least one optical element may consist of a single optical element made of glass, and the predetermined medium is the air.
In particular, the single element is a parallel plate.
Alternatively, the single element may be a right-angle prism, the beam splitting surface being provided on an inclined surface of the right-angle prism.